The invention relates to aircraft ground proximity warning systems and in particular to warning systems utilizing a glide slope radio beam wherein a warning is generated when the aircraft's descent below glide slope exceeds a predetermined value for a particular altitude.
In the prior art, specifically the U.S. Pat. application of Bateman Ser. No. 480,727, entitled "Aircraft Ground Proximity Warning Instrument," filed on June 19, 1974 and assigned to the assignee of this application, a warning is generated whenever the aircraft is not in a safe zone of operation below a predefined altitude such as 650 feet above ground level. This safe zone of operation, or the distance below the glide slope, will vary depending upon the aircraft's altitude above ground. For example, between 650 feet and 150 feet above ground the aircraft must stay within a predefined angular distance, measured in dots, of the glide slope or a warning signal will be triggered. This feature allows the pilot to adjust the absolute altitude of the aircraft with somewhat greater latitude at greater altitudes where such maneuvers are inherently safer.
However, there are certain conditions under which the crew of an aircraft may wish to descend below the glide slope sufficiently to trigger the warning where such a descent would be in conformity with safe operating procedures. For example, there are a number of runways where the glide slope beam is substantially displaced from the threshold toward the middle of the runway. Thus, under visual operating conditions, it is not unusual for the crew of an aircraft to make use of this additional runway which in turn necessitates descending sufficiently below the glide slope to trigger a warning. A warning triggered under these conditions, especially at higher altitudes such as 650 feet, can be considered advisory at best and as a practical matter most crew members will consider it a nuisance warning with a resultant loss in credibility of the system as a whole.
However, in a majority of cases it is still considered desirable to have the warning envelope of the same general type provided by the system in Bateman 480,727 so that the aircraft's crew may be apprised of a potentially dangerous descent under the glide slope even at these greater altitudes.